媒介
在写这个promise之前,愿望你已对es6中的Promise很熟悉了,概念性和基础的东西就不再讲了,不懂的同砚可以去看看阮一峰先生的es6教程. 我主要按以下5个步骤来一步一步完成,异步的完成我放在了背面,所以前面几步暂不斟酌
- 完成一个基础的MyPromise
- 完成then的链式挪用
- 处置惩罚reolve函数的参数是MyPromise实例的状况以及处置惩罚then要领中前一个回调函数返回的也是一个MyPromise实例的状况
- 完成异步的MyPromise
- 完美MyPromise的别的要领
1.完成一个基础的MyPromise
/*
* 这里我将promise的3个状况离别定义为: pending, resolved, rejected
* 个中fn必需是个函数, 必需经由过程new来运用
*/
function MyPromise(fn) {
if (!(this instanceof MyPromise)) {
throw new TypeError('MyPromise must be constructed via new');
}
if (typeof fn !== 'function') {
throw new TypeError('MyPromise constructor argument is not a function');
}
this.state = 'pending'; // 出初始化状况
this.value = undefined; // 初始化一个值, 用来存储resolve或许reject的值
// 实行 fn 要领
executeFn(fn, this);
}
MyPromise.prototype.then = function(onFullfilled, onRejected) {
var res = undefined;
var cb = this.state === 'resolved' ? onFullfilled : onRejected;
res = cb(this.value);
}
// 实行 fn 要领
function executeFn(fn, promise) {
var done = false; // 声明一个变量, 防备resolve, reject一连挪用
try {
fn(function _resolve(value) {
if(done) return;
done = true;
resolve(promise, value);
}, function _reject(reason) {
if(done) return;
done = true;
reject(promise, reason);
});
}
catch(err) {
if(!done) {
done = true;
reject(promise, err);
}
}
}
function resolve(promise, value) {
promise.state = 'resolved';
promise.value = value;
}
function reject(promise, error) {
promise.state = 'rejected';
promise.value = error;
}
如许就完成了一个基础版的MyPromise,挪用要领同Promise,以下:
new MyPromise((resolve, reject) => {
// resolve('resolved');
reject('rejected');
}).then(res => {
console.log('>>> res', res);
}, err => {
console.log('>>> err', err);
});
2.完成then的链式挪用
原生Promise支撑链式挪用,而且then要领会返回一个新的Promise实例,第一个回调函数完成今后,会将返回效果作为参数,传入第二个回调函数,所以我们可以修改下then要领,其他稳定
MyPromise.prototype.then = function(onFullfilled, onRejected) {
var self = this;
var res = undefined;
var cb = this.state === 'resolved' ? onFullfilled : onRejected;
var newPromise = new MyPromise(function(_resolve, _reject) {
try {
res = cb(self.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
return newPromise;
}
如许的话,链式挪用也就完成了,测试了下,没啥题目
new MyPromise((resolve, reject) => {
resolve('resolved');
// reject('rejected');
}).then(res => {
console.log('>>> res', res);
return 'res1';
}, err => {
console.log('>>> err', err);
return 'err1';
}).then(res2 => {
console.log('>>> res2', res2);
}, err2 => {
console.log('>>> err2', err2);
});
3. 处置惩罚reolve函数的参数是MyPromise实例的状况以及处置惩罚then要领中前一个回调函数返回的也是一个MyPromise实例的状况
resolve函数的参数除了一般的值之外,还多是另一个MyPromise实例,以下,这个时刻p1的状况决议了p2的状况
const p1 = new MyPromise(function(resolve, reject) {
// ...
});
const p2 = new MyPromise(function(resolve, reject) {
// ...
resolve(p1);
});
一样,在then要领中,前一个回调函数有可以返回的也是一个MyPromise对象,这时候后一个回调函数就会守候该MyPromise对象的状况发作变化,才会被挪用,比方:
const p1 = new MyPromise(function(resolve, reject) {
// ...
});
const p2 = new MyPromise(function(resolve, reject) {
// ...
resolve('p2 resolved');
});
p2.then(res1 => {
console.log('>>> res1', res1);
return p1;
}, err1 => {
console.log('>>> err1', err1);
}).then(res2 => {
console.log('>>> res2', res2);
}, err2 => {
console.log('>>> err2', err2);
})
相识以上后,我们来完成它
function resolve(promise, value) {
if(!handlePromise(promise, value)) return; // 增添这行
promise.state = 'resolved';
promise.value = value;
}
/*
* 增添一个函数用来处置惩罚返回值或许resolve的参数是promise的状况, 末了的返回值起个标识作用
*/
function handlePromise(promise, value) {
if(value === promise) {
reject(promise, 'A promise cannot be resolved with itself');
return;
}
if(value && (typeof value === 'object' || typeof value === 'function')) {
var then = value.then;
if(typeof then === 'function') {
executeFn(then.bind(value), promise);
return;
}
}
return true;
}
// 同时then中增添一行代码
MyPromise.prototype.then = function(onFullfilled, onRejected) {
var self = this;
var res = undefined;
var cb = this.state === 'resolved' ? onFullfilled : onRejected;
var newPromise = new MyPromise(function(_resolve, _reject) {
if(self.state === 'pending') return; // 增添这行
try {
res = cb(self.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
return newPromise;
}
到这里,就处理了以上2个题目,测试一下
const p = new MyPromise((resolve, reject) => {
// resolve('resolve a promise');
reject('reject a promise');
});
// resolve参数是MyPromise实例
new MyPromise((resolve, reject) => {
resolve(p);
// reject('rejected');
}).then(res => {
console.log('>>> res', res);
return 'res1';
}, err => {
console.log('>>> err', err);
return 'err1';
}).then(res2 => {
console.log('>>> res2', res2);
}, err2 => {
console.log('>>> err2', err2);
});
// then第一个要领返回的是MyPromise实例
new MyPromise((resolve, reject) => {
resolve('resolved');
// reject('rejected');
}).then(res => {
console.log('>>> res', res);
return p;
}, err => {
console.log('>>> err', err);
return 'err1';
}).then(res2 => {
console.log('>>> res2', res2);
}, err2 => {
console.log('>>> err2', err2);
});
4. 完成异步的MyPromise
经由前面3步,MyPromise的已完成了泰半,接着我们来完成异步的MyPromise. 既然是异步,我们并不知道它什么时刻完毕,然则我们可以将它的异步回调存入一个数组,待它完毕后实行它,好吧,实在就是观察者形式了
首先在组织函数中加上一句
function MyPromise(fn) {
// ...这里省略,加上下面这行
this.callbacks = []; // 存储异步的回调要领
// 实行 fn 要领
executeFn(fn, this);
}
然后在resolve和reject要领中离别加上
function resolve(promise, value) {
// ...这里省略,加上下面几句
promise.callbacks.forEach(function(cb) {
cb();
});
}
function reject(promise, error) {
// ...这里省略,加上下面几句
promise.callbacks.forEach(function(cb) {
cb();
});
}
末了在then要领中,将异步的回调发存入数组中
MyPromise.prototype.then = function(onFullfilled, onRejected) {
// ...这里省略,稳定
var newPromise = new MyPromise(function(_resolve, _reject) {
// 状况为pending时,将回调存入数组,由于then中要领也是异步实行
// 所以用setTimeout,同时直接return
if(self.state === 'pending') {
self.callbacks.push(function() {
setTimeout(function() {
// 这里须要再次推断
cb = self.state === 'resolved' ? onFullfilled : onRejected;
try {
res = cb(self.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
});
return;
}
// then中是异步实行
setTimeout(function() {
try {
res = cb(self.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
});
return newPromise;
}
到这里,异步的MyPromise也就完成了,then要领代码有点乱,我们整顿下
MyPromise.prototype.then = function(onFullfilled, onRejected) {
// 这里删除了一部份代码
var self = this;
var newPromise = new MyPromise(function(_resolve, _reject) {
if(self.state === 'pending') {
self.callbacks.push(function() {
// 同时将这部份的代码抽成了以下要领
handleResolved(self, onFullfilled, onRejected, _resolve, _reject);
});
return;
}
handleResolved(self, onFullfilled, onRejected, _resolve, _reject);
});
return newPromise;
}
function handleResolved(promise, onFullfilled, onRejected, _resolve, _reject) {
setTimeout(function() {
var res = undefined;
var cb = promise.state === 'resolved' ? onFullfilled : onRejected;
// 须要对cb举行推断
if(typeof cb !== 'function') {
if(promise.state === 'resolved') {
_resolve(promise.value);
}
else {
_reject(promise.value);
}
return;
}
try {
res = cb(promise.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
}
测试以下, 固然没啥题目了
const p = new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('resolve a promise');
// reject('reject a promise');
}, 2 * 1000);
});
new MyPromise((resolve, reject) => {
resolve(p);
// reject('rejected');
}).then(res => {
console.log('>>> res', res);
return 'res1';
}, err => {
console.log('>>> err', err);
return 'err1';
}).then(res2 => {
console.log('>>> res2', res2);
}, err2 => {
console.log('>>> err2', err2);
});
到这里,一个也许的MyPromise也就基础完成完成了,整顿后的完全代码以下
function MyPromise(fn) {
if (!(this instanceof MyPromise)) {
throw new TypeError('MyPromise must be constructed via new');
}
if (typeof fn !== 'function') {
throw new TypeError('MyPromise constructor argument is not a function');
}
this.state = 'pending'; // 初始化状况
this.value = undefined; // 初始化一个值, 用来存储resolve或许reject的值
this.callbacks = []; // 存储异步的回调要领
// 实行 fn 要领
executeFn(fn, this);
}
MyPromise.prototype.then = function(onFullfilled, onRejected) {
var self = this;
var newPromise = new MyPromise(function(_resolve, _reject) {
if(self.state === 'pending') {
self.callbacks.push(function() {
handleResolved(self, onFullfilled, onRejected, _resolve, _reject);
});
return;
}
handleResolved(self, onFullfilled, onRejected, _resolve, _reject);
});
return newPromise;
}
function handleResolved(promise, onFullfilled, onRejected, _resolve, _reject) {
setTimeout(function() {
var res = undefined;
var cb = promise.state === 'resolved' ? onFullfilled : onRejected;
if(typeof cb !== 'function') {
if(promise.state === 'resolved') {
_resolve(promise.value);
}
else {
_reject(promise.value);
}
return;
}
try {
res = cb(promise.value);
_resolve(res);
}
catch(err) {
_reject(err);
}
});
}
// 实行 fn 要领
function executeFn(fn, promise) {
var done = false; // 声明一个变量, 防备resolve, reject一连挪用
try {
fn(function _resolve(value) {
if(done) return;
done = true;
resolve(promise, value);
}, function _reject(reason) {
if(done) return;
done = true;
reject(promise, reason);
});
}
catch(err) {
if(!done) {
done = true;
reject(promise, err);
}
}
}
function resolve(promise, value) {
if(!handlePromise(promise, value)) return;
promise.state = 'resolved';
promise.value = value;
promise.callbacks.forEach(function(cb) {
cb();
});
}
function reject(promise, error) {
promise.state = 'rejected';
promise.value = error;
promise.callbacks.forEach(function(cb) {
cb();
});
}
// 用来处置惩罚返回值或许resolve的参数是promise的状况, 末了的返回值起个标识作用
function handlePromise(promise, value) {
if(value === promise) {
reject(promise, 'A promise cannot be resolved with itself');
return;
}
if(value && (typeof value === 'object' || typeof value === 'function')) {
var then = value.then;
if(typeof then === 'function') {
executeFn(then.bind(value), promise);
return;
}
}
return true;
}
5. 末了来完成下MyPromise的别的要领
MyPromise.prototype.catch = function(onRejected) {
return this.then(null, onRejected);
}
// 只需不是pending状况都邑实行
MyPromise.prototype.finally = function(cb) {
return this.then(
function(value) {
return MyPromise.resolve(cb()).then(function() {
return value;
});
},
function(err) {
return MyPromise.resolve(cb()).then(function() {
throw err;
});
}
);
}
MyPromise.resolve = function(val) {
return new MyPromise(function(resolve, reject) {
resolve(val);
});
}
MyPromise.reject = function(err) {
return new MyPromise(function(resolve, reject) {
reject(err);
});
}
/*
* all要领用于将多个 MyPromise 实例,包装成一个新的 MyPromise 实例
* 只要悉数实例都resolved,才会resolve; 只需个中一个rejected,就会reject
* 参数可以不是数组,但必需具有 Iterator 接口, 同时内里的值可以也不是promise实例
*/
MyPromise.all = function(promiseArr) {
var args = [].slice.call(promiseArr);
return new MyPromise(function(resolve, reject) {
var arr = [];
var resolveCount = 0;
var argsLen = args.length;
for(var i = 0; i < argsLen; i++) {
handle(i, args[i]);
}
function handle(index, val) {
MyPromise.resolve(val).then(function(value) {
arr[index] = value;
if(++resolveCount === argsLen) {
resolve(arr);
}
}, reject);
}
});
}
/*
* race要领与all要领相似,只需个中一个实例状况发作转变resolved / rejected即可
* 参数可以不是数组,但必需具有 Iterator 接口, 同时内里的值可以也不是promise实例
*/
MyPromise.race = function(promiseArr) {
var args = [].slice.call(promiseArr);
return new MyPromise(function(resolve, reject) {
for(var i = 0; i < args.length; i++) {
MyPromise.resolve(args[i]).then(resolve, reject);
}
});
}
至此Promise的完成就算完成了,完全代码的地点点这里
末了
说点题外话,在口试的过程当中,经常会碰见口试官请求现场完成一个Promise,看了这篇文章后愿望对你有所协助,已可以完成一个Promise了,而关于口试中其他的promise的相干题目可以轻松应对